The STT electric arc welder sold by The Lincoln Electric Company of Cleveland, Ohio has been used for pipe welding where a molten metal from an advancing welding wire is deposited into a weld puddle in an open root joint between two juxtapositioned ends of adjacent pipe sections. The open root joint extends around the pipes in a circular path, which path may vary somewhat by axial deviations. The joint between pipe sections is formed by converging walls terminating in generally parallel walls spaced from each other to define a separating gap, known as an open root. The gap between the ends of the pipe sections, or plates, is not a fixed distance so the welder as it progresses around the open root encounters a gap with a variety of spacings. As disclosed in Stava U.S. Pat. No. 5,001,326, the STT welder has a contact holder though which the advancing wire passes so the welding current can be directed through the holder to the advancing wire and then into the molten weld puddle formed between the ends of the pipe sections. This puddle bridges the gap to form the root bead of the pipe welding operation. The STT welder uses a high frequency switching power supply to create a distinct short circuit current wave shape. This frequency relates to the operating frequency of the power supply and not to the frequency of the weld process. The electrode stick out or extension in the welding field is the distance between the contact holder and the weld puddle. As the gap between the ends of the plates formed by the adjacent pipe sections vary, the puddle formed by the first pass of the electric arc welding in a pipe welding operation changes shape. This change presents known problems in a pipe welding process.
As disclosed in Stava U.S. Pat. No. 5,001,326, the objective of the high frequency switching power supply known as the STT electric arc welder is to maintain a uniform volume in the molten metal ball on the end of the electrode as the wire advances toward the molten puddle. This prior patent discloses how the control system of the STT can be used to detect and measure the actual wattage forming the molten metal ball on the end of the electrode. This wattage is set for an extension and is maintained from one welding cycle to the next. As the stick out of the electrode changes, the welding current is adjusted to maintain a constant wattage based upon a preset reference wattage obtained at the start of the welding operation. In this manner a constant sized ball is created during each welding cycle. When using the STT electric arc welder, if the welding current is not changed to compensate for changes in the electrode extension or stick out, the shorting frequency will change. This changing frequency is not the switching frequency of the power supply, but is the weld shorting frequency. As the electrode extension (stick out) is reduced, the volume of molten metal in the advancing ball is reduced and the arc length is slightly reduced. This reduction in the size of the molten metal ball causes the time between short circuits in the welding operation to be reduced. This increases the shorting frequency, as determined by the number of shorts per unit time. Conversely, if the electrode extension or stick out is increased, the heating of the electrode is increased. If there is no current adjustment, the volume of molten metal on the end of the advancing wire increases. This action slightly increases the arc length to increase the time between short circuits to decrease the shorting frequency of the welder. The frequency change during use of the STT for pipe welding has not been used as a measuring parameter.
The STT is used for pipe welding to allow manual control of penetration and heat into the puddle; however, it does not solve the problem experienced with variations in the width of the gap when laying the first bead in an open root joint. Stava U.S. Pat. No. 5,001,326 does teach a procedure for measuring the stick out during use of an STT welder for use in maintaining a given ball size. While the electrode or welding wire is short circuited, the voltage drop across the shorted electrode is measured and recorded. The value of the welding current is essentially fixed during this measurement. Consequently, the sensed or measured voltage is directly proportional to the stick out or extension of the welding wire. This measured extension voltage is averaged over several cycles and then stored in a memory, such as a capacitor. In the Stava patent, this stored value is multiplied by the peak current value during subsequent welding cycles. The result of this multiplication is a measurement of the watts during each welding cycle. Calculated wattage is stored in memory, such as a capacitor, and is referred to as a "reference wattage." This reference wattage is obtained at the start of the welding process based upon an adjusted stick out. Thereafter, the reference wattage is used for comparison with the instantaneous wattage. A control software program holds the instantaneous wattage at the reference wattage. This total control scheme, as shown in Stava U.S. Pat. No. 5,001,326, is used to maintain a fixed volume of molten metal on the end of the wire as it is advancing toward puddle during the short circuit condition, irrespective of changes in the stick out or extension. There is no teaching of changing the stick out during a welding process to control the welding process. There is no teaching of changing the heat generated during a single pass. Stick out or extension of the advancing wire is used to maintain a constant wattage during the wire melting portion of the welding cycle.
The use of a high frequency switching power supply such as the STT electric arc welder for pipe welding is well known. However, when laying the root bead during the first pass in the open root joint between the pipe sections, the power supply can not automatically compensate for changes in the gap between the pipe sections. When the gap is wide, the puddle falls through the gap. When the gap is narrow, the puddle does not penetrate the gap. Thus, manual intervention is required with the resulting variations. There is a need for controlling the welding operation during the initial bead in the open root joint of the type encountered in pipe welding or other heavy plate welding.
The STT welder sold by The Lincoln Electric Company is a short circuit arc welder which is operated by a high frequency switching network to create a current curve as shown in FIG. 4 of Stava U.S. Pat. No. 5,001,326. This welder is commercially successful; however, it does not have the same capabilities of a constant voltage welder wherein the weld puddle temperature can be controlled by merely changing the wire stick out or electrode extension. This is a limitation of a welder for creating the STT current curve. Consequently, puddle temperature, as important in pipe welding, is adjusted when using an STT welder by intervention or presetting of the weld currents. Even with this limitation, the STT welder is superior to a constant voltage weld which creates excessive puddle heat.